Decadal Contributions of Fire Disturbance to the Carbon Balance of Boreal Ecosystems Using a Process-Based Global Dynamic Vegetation Model

Abstract:

Fires are an intrinsic disturbance factor over the boreal regions. Fire impacts ecosystem biophysical properties and carbon stocks, and over long time scales, species composition and fires have strong interactions. The contemporary carbon sink in the Northern Hemisphere is partly related to the forest recovery from the legacy of historical fire disturbances. However very little is known on the relative contributions to the current-day carbon sink from past fires that occurred at different times in the recent history. In the present study we develop a novel conceptual approach to quantify the decadal contributions of fires during the 20th century to the carbon balance of the early 21st century, by modelling the legacy sink created by decadal cohorts of past fires. This quantification is realized by conducting a suite of simulations using the ORCHIDEE global dynamic vegetation model, in which the prognostic SPITFIRE fire module was turned off in each decade sequentially during the 20th century, and turned before and after, to account for carbon storage recovery. The vegetation dynamics interact with fires via the dynamic vegetation module of ORCHIDEE, in which fire impacts vegetation mortality and tree-grass competition. We found that the existence of fires reduces the forest coverage in the transitional forest-grassland zone around 45^oN-50^oN region over central Asia. In this region, the existence of recurrent fire acts to lower equilibrium biomass and soil carbon stocks. Historical fire disturbances and the resulting forest successional dynamics contribute a legacy to the boreal carbon sink during the period 2010-2011, with the lagged sink effects of fires that happened in the recent three decades accounting for over 70% of the total fire-induced sink during 2010-2011. Most of current sink from past fires is contributed by fires with moderate frequency with fire return intervals of 10-100 years.